October 18, 1901.] 



SCIENCE 



589 



the light, thus making the effect of the gas 

 action and light pressure in the same di- 

 rection on one vane, and in opposite direc- 

 tions on the other, (3) The pressure of the 

 air in the bell jar was varied, and the 

 pressures were chosen in the vicinity of 

 that pressure at which the gas action was 

 very small. (4) The length of exposure of 

 light on the vane in most of the observa- 

 tions was short. The gas action, which be- 

 gins at zero and increases with the length 

 of exposure, was thus reduced in compari- 

 son with the instantaneous action of the ra- 

 diation pressure. By means of an inclined 

 glass plate placed in front of the aperture a 

 portion of the incident light was thrown on 

 a thermopile. The deflection of a galva- 

 nometer connected with the latter gave the 

 relative light intensities. 



Two methods of determining radiation 

 pressure were used : 



(1) The vane was exposed continuously 

 to the light until the turning points of the 

 vibration of the balance showed that static 

 conditions had been reached. The other 

 vane was then exposed. Finally the whole 

 suspended system was turned through 180°, 

 and the vanes were exposed in turn. The 

 mean of the angles of deflection, multiplied 

 by the torsion coefficient of the fiber and 

 divided by the lever arm, gave the force in 

 dynes acting on the vane. (2) The vanes 

 were exposed for a quarter of the period of 

 the suspended system. The period, damp- 

 ing coefficient, torsion coefficient and lever 

 arm being known, the value of the radia- 

 tion could be found. The two methods 

 gave practically the same result except for 

 the air pressures for which the gas action 

 was large. 



The energy falling upon the vanes was 

 measured by means of a bolometer consist- 

 ing of a thin disc of platinum, about the 

 size of the vanes, covered with platinum 

 black. The bolometer, occupying exactly 

 the position which the glass vane had pre- 



viously occupied, was made one of the arms 

 of a Wheatstone bridge. The bridge was 

 balanced, the bolometer exposed to the 

 light, and the throw of the bridge galvanom- 

 eter read. Later the disc was heated by an 

 electric current which entered and left at 

 two equipotential points on the bridge cur- 

 rent and the galvanometer throw again was 

 read. The current strength and the resist- 

 ance of the disc being known, the activity of 

 the disc is given by rR x 10^ ergs per sec- 

 ond. The readings of the thermopile be- 

 fore mentioned made it possible to reduce all 

 observations to a constant light intensity. 

 If ^ = energy per second falling upon a 

 surface, a = the percentage of the radiation 

 reflected, v =■ the velocity of light, then, 

 theoretically, the value of the radiation 



(i + a)E 

 pressure is . The experimental 



value of the pressure as found by the 

 authors was about 80 per cent, of the theo- 

 retical value. 



8. ' The Absorption Spectrum of Ferric 

 Hydrate ': B. E. Moore, University of Ne- 

 braska. 



The author in a recent article upon ^ A 

 Spectrophotometric Study of the Hydrolysis 

 of Ferric Chloride,' has pointed out that 

 the final product is colloid ferric hydrate. 

 This product can be formed by Graham's 

 dialytic method. It would then be inferred 

 that identical products would have identical 

 absorption spectra. 



It was found that the absorption of the 

 two products was quite different in charac- 

 ter. The addition of given quantities of 

 chlorine or of hydrochloric acid, did not 

 effect any transformation in the dialyzed 

 colloid, i. e., crystalloids, if present, were 

 not changed by this treatment. If dialyzed 

 ferric hydrate, or hydrolyzed ferric chloride 

 were added to fresh dilute ferric chloride, 

 the hydrolysis of the latter was accelerated. 

 The greater acceleration was caused by the 

 hydrolyzed ferric chloride. From this it is 



